The fertilization of animals by artificial insemination (AI) and embryo transplant following in vitro fertilization is an established practice. In the livestock production industry, the ability to influence the reproductive outcome toward offspring having one or more desired characteristics has obvious advantages. By way of example, there would be an economic benefit in the dairy industry to preselect offspring in favor of the female sex to ensure the production of dairy cows. The separation of sperm into enriched populations of X and Y chromosome-bearing cells, known as gender enriched semen or gender enriched sperm, is one method of achieving preselected offspring.
Johnson et al. (U.S. Pat. No. 5,135,759) describe the separation of intact X and Y chromosome-bearing sperm populations according to DNA content using a flow cytometer/cell sorter into X and Y chromosome-bearing sperm enriched populations. As described, the sperm is combined with a DNA selective dye at a temperature of 30° C. to 39° C. for a period of 1 hour (39° C.) to 1.5 hours (30° C.). A flow cytometer is then used to measure the amount of fluorescent light given off when the sperm passes through a laser beam. Because the X chromosome-bearing sperm contains more DNA than the Y chromosome-bearing sperm, approximately 3% to 5% depending upon the species, the X chromosome-bearing sperm yields a greater intensity of fluorescent light than the Y chromosome-bearing sperm. Droplets containing single sperm of a predetermined fluorescent intensity are given a charge and electrostatically deflected into collection vessels. The collected, gender enriched sperm population, is then used for microinjection or artificial insemination. Notably, this method requires that the sperm cells be physically sorted to achieve the gender enriched sperm population. Physically sorting according to Johnson requires time and cost.